Align link configuration with the needs of the Maven Javadoc Plugin

The -link parameters passed into the javadoc executable are used
as-is. For example, redirects are not followed. Unlike 3.x, 2.x of the
Maven javadoc plugin does not attempt to follow redirects before links
are passed to the javadoc executable. As a result, for external links
to work, the links that we provide must be canonical and produce a
200 response when accessed.

Closes gh-14031

Author: wilkinsona

spring-boot-docs/pom.xml

@@ -785,13 +785,13 @@
 									<quiet>true</quiet>
 									<stylesheetfile>${basedir}/src/main/javadoc/spring-javadoc.css</stylesheetfile>
 									<links>
-										<link>http://docs.oracle.com/javase/7/docs/api/</link>
-										<link>http://docs.oracle.com/javaee/7/api/</link>
-										<link>http://docs.spring.io/spring-framework/docs/${spring.version}/javadoc-api/</link>
-										<link>http://docs.spring.io/autorepo/docs/spring-security/${spring-security.version}/apidocs/</link>
-										<link>http://tomcat.apache.org/tomcat-8.0-doc/api/</link>
+										<link>https://docs.oracle.com/javase/7/docs/api/</link>
+										<link>https://docs.oracle.com/javaee/7/api/</link>
+										<link>https://docs.spring.io/spring-framework/docs/${spring.version}/javadoc-api/</link>
+										<link>https://docs.spring.io/autorepo/docs/spring-security/${spring-security.version}/apidocs/</link>
+										<link>https://tomcat.apache.org/tomcat-8.0-doc/api/</link>
 										<link>https://www.eclipse.org/jetty/javadoc/${jetty.version}/</link>
-										<link>http://www.thymeleaf.org/apidocs/thymeleaf/${thymeleaf.version}/</link>
+										<link>https://www.thymeleaf.org/apidocs/thymeleaf/${thymeleaf.version}/</link>
 									</links>
 								</configuration>
 							</execution>